1. Technical Field
The present disclosure relates to prosthetic implants and more specifically, prosthetic implants that include polymer material for fixation of the implant to bone and fixation between implant components.
2. Related Art
During joint replacement surgery, also referred to as replacement arthroplasty, a joint implant is inserted into or otherwise attached to a bone that has been prepared to receive the implant, and the implant is secured. Reliable stabilization, or fixation, is essential for the success of joint replacement. Movement of the implant relative to the bone often results in formation of a fibrous interface between the implant and the bone. The fibrous interface may cause further loosening and, ultimately, destabilization of the implant, thereby necessitating additional surgery or surgeries, commonly referred to as corrective or revision surgeries.
Several methods of fixating joint implants, or components of joint implants, on or in a bone are known. One method of stabilization is to permanently affix the joint implant to the bone using a bone cement. Stabilization with bone cement requires the drilling of oversized holes in the bone, which are filled with the cement prior to insertion of the joint implant. The implant is inserted into the cement-filled bone and allowed to harden. Unfortunately, implants that have been cemented are prone to loosening and are extremely difficult to remove when replacement is required. Furthermore, the cementing process requires difficult preparation of the bone surface.
Other methods of stabilizing a joint implant are cementless methods, which include stabilization by interference or press fit, stabilization by various structures, and other methods. For example, various stabilizing devices or structures, such as pegs, screws, or fins, protrude from the implant and are used to attach the prosthetic joint to the bone. The main disadvantage of this type of cementless method is that the protruding structures frequently create stress patterns in the bone. These stress patterns produce undesirable bone remodeling that can lead to destabilization of the implant. In another type of cementless method, often referred to as biological fixation, the implants are covered, coated, or enveloped with a porous surface material, such as a polymer or a ceramic material that allows bone growth into the surface of the implant. This growth, commonly referred to as “bone ingrowth”, stabilizes the implant. While less prone to the destabilization problems encountered when implants are stabilized by other methods, implants stabilized by biological fixation must remain completely secure for three to eight weeks after the implant surgery to allow sufficient bone ingrowth to occur. Any movement of the implant within the bone during these first few weeks after surgery results in the formation of a fibrous interface between the implant and the bone that prevents bone ingrowth and the desired stabilization effect.
When a prosthetic joint deteriorates, loosens, destabilizes, or otherwise becomes problematic, the joint must be removed and replaced with a new joint during a subsequent (or revision) surgery. Stabilization of a new joint implant during a revision surgery is particularly challenging. As a result of the prior removal of large amounts of bone tissue during the first joint replacement surgery, cavities often exist between the new implant and the bone to which the new implant is being attached, thereby making it impossible to achieve a tight fit between the new implant and the bone. It is advantageous to fill these cavities and provide adjunct stabilization of the implant relative to the bone for several reasons. First, undesirable bone remodeling may occur if stress distribution after revision surgery changes. Thus, it is desirable to maintain the load transfer in a bone after revision surgery as similar as possible to that existing with the primary implant. Second, it is desirable to distribute stress patterns in the bone to help bone reconstitution and avoid risk of bone fracture. Third, the filling of cavities helps minimize stress levels in the implant itself, thereby reducing the risk of implant fracture. Additionally, when utilizing a biologically stabilized implant, the cavities should be filled to avoid undesirable movement of the implant relative to the bone, thereby substantially reducing the formation of a fibrous interface to allow proper bone ingrowth and permanent fixation of the implant. Moreover, cavities should be filled to substantially reduce the exposure of unprotected bone tissue. Exposed bone tissue is susceptible to infection and is accessible to harmful polymer particles, which often form as a result of the “shedding” of the polymer-coated, articulating surfaces of joint implants. These polymer particles may cause localized osteolysis, a time-dependent process that arises from an inflammatory reaction caused by the particulate debris of polymer coatings composed of polymers such as polyethylene.
One particularly challenging joint implant stabilization scenario is found in hip replacement revision surgery. During this surgery, the proximal aspect of the femur resembles the shape of an ice cream cone as a result of previous surgeries. Therefore, during revision hip replacement surgery, it is particularly challenging to adjust both the distal and proximal aspects of the femur to fit the implant. When a distally fixed hip implant is installed into a femoral canal, the implant passes through the ice cream cone-shaped proximal aspect of the femur and is secured to a distal aspect of the femur. Due to the shape, a cavity often remains between the implant and the cortical bone tissue in the proximal aspect of the femur. It is desirable to fill the cavity to distribute some of the stress between the distal and proximal femur in order to promote bone reconstitution in the proximal femur. In addition, the filling of the cavity provides additional stabilization of the implant, thereby decreasing the risk of it becoming loose.
Another challenging scenario is an example of hip revision surgery when an acetabulum needs reamed excessively until a substantial portion of it is hemispherical in order to install an acetabular component of a hip prosthesis. Often, achieving a desired shape is still impossible, and a non-optimal bi-lobed configuration of an acetabular component is utilized, such as that in the acetabular components offered by DePuy Orthopaedics (Warsaw, Ind.), or Johnson & Johnson, (New Brunswick, N.J.), or a revision acetabular cage is used. During revision knee replacement arthroplasty, femoral or tibial components are often combined with metal augments, which are wedges or blocks of metal to make up for the lost bone and fill the gaps.
However, the shape of the cavities between a joint implant and a bone is often irregular and cannot be filled by standardized metal implants. To circumvent this problem, the technique of allografting, or packing allograft bone into bone cavities, followed by the introduction of bone cement, is often utilized. The allograft bone is crushed, morselized, or fashioned into shapes suitable for packing the cavity, the shapes are packed into the cavity, and bone cement is added. This allografting technique is prone to all of the problems described above that are associated with cementing methods. Specifically, during any subsequent revision surgeries, removal of the allograft and additional bone tissue is required. This cumulative bone loss creates a natural limit to a number of revisions that can be performed. Restoration of bone tissue, or stock, is limited or does not occur when allografts are used. Moreover, bone necrotization may occur and persist in the allografts.
Morselization of the allograft material may promote bone remodeling and restoration by causing the release of growth factors present in the graft, and the morselized material may be impacted to make it easier for the ingrowing bone to climb up into the graft. However, when allograft bone is being packed into the cavity, significant force is utilized. The use of such force increases risk of bone fracture and trauma. If the allograft material is not morselized, but is fashioned into preformed shapes, the shapes available often do not fit properly into the cavity to be filled. Finally, the use of allografts, in general, carries increased risks of disease transmission and graft rejection. One alternative to allografting is to collapse the adjacent bone around the implant and cable the bone onto the implant. However, this procedure has been associated with bone degradation.
Permanent, such as, metallic fixation devices, must be removed at the time of revision surgery. In the case of screws, there are times when they are difficult to remove, which is time consuming to the surgeon. An added disadvantage is that such devices, particularly screws, may fracture, with the resulting remnants causing tissue damage. Also available are spikes, pegs, or fins, or any combination thereof, that are driven into the bone, for example, for fixation around the periphery or the dome of acetabular cups, however, upon their removal, a hole or cavity is formed that still must be filled with bone graft during revision surgery.
A modular peg is currently available for fixation of an acetabular component of a hip implant. The modular peg can be inserted for fixation after the acetabular cup was implanted. Due to it being broad compared to the root diameter of a screw, the peg provides better rotational stability than the screw. Moreover, the peg seals a connection between the acetabular cup and the peg, thereby substantially reducing exit of debris through this connection. During revision surgery, the peg can be removed prior to removal of the cup, which allows curved osteotomies, or gouges, to be passed around the outside of the acetabular shell during surgery, thus simplifying the procedure. Still, the modular peg results in a cavity in an acetabular bed that requires packing with bone graft or other materials.
Temporary fixation structures manufactured of resorbable, degradable, or temporary, materials are gradually resorbed by the tissue after the installation. The resulting bone cavity is gradually replaced by re-growing bone. A variety of biodegradable devices for temporary fixation of joint implants are available, all of which, however, suffer from a variety of shortcomings. Bone screws manufactured of materials that are resorbed by the body are available. While they do not require removal after the need for fixation passes, they are of limited mechanical strength. When bone screws are used for fixation of an acetabular component of a hip implant, fixation is often lost prematurely, thus not allowing adequate time for the bone ingrowth to occur, and resulting in destabilization of the implant on the bone. In general, bone screws, including those made of resorbable materials, are known to back out of the bone, press against polymer components of a joint implant, create dents, and generate polymer particles. Moreover, degradation, or resorption of the screw material occurs faster than bone re-growth, thus leaving exposed cavity, which is also prone to osteolysis, particularly in the cases where polymeric prosthetic surfaces are employed that may generate particles during operation of the prosthesis. Generally, a challenge is to adjust the rate of degradation of such a temporary fixation device to correspond to the rate of bone tissue regrowth, thereby substantially reducing the presence of a bone cavity, or exposed bone tissue, prone to infection or osteolysis. Covers, or seals, to cover screw holes in the bone, are available, however, these require additional fitting and installation steps during surgery.
In general, many of the currently available methods and devices for adjunct stabilization of implants, such as impaction allografting or the collapsing and cabling of bone, require the presence of sufficient amounts of high quality bone tissue in the bone to which the implant is being attached. When bone tissue is lost, due to disease or a pathological condition or for other reasons, the constructs become unstable. Persons with thin or fragile bones, such as osteoporosis patients, avascular necrosis patients, and patients with metastatic bones, are especially in need of joint implants. However, their bone tissue is often not sufficiently strong for the stabilization of joint implants without adjunct fixation. Therefore, currently available adjunct stabilization devices and methods fail to satisfy the requirements of patients who are most in need of joint implants.
Thus, there is a need for systems and devices that provide reliable adjunct stabilization of joint implants yet allow for reestablishment of bone, bone ingrowth, or restoration of bone stock. Particularly, there is a need for methods that provide additional stabilization of implants in tubular bones. Specifically, implants are needed that permit and promote bone regrowth in the cavities between the implant and the bone cortex. This need is particularly urgent during revision arthroplasties, such as revision hip replacement arthroplasty, when a large amount of bone tissue has been removed during prior surgeries and it particularly challenging to tightly fit an implant into a bone without leaving a bone cavity, or in situations when disease and pathological conditions reduce the amount and quality of bone tissue available for fitting.
Devices and systems for adjunct stabilization are needed that allow for bone restoration, are easily adaptable to a variety of local conditions, reliably stabilize the implant, maintain the load transfer, and distribute stress patterns in the bone in a manner that promotes bone reconstitution, reduces the risk of bone fracture, reduces stress levels in the implant itself, and reduces the risk of implant fracture. Suitable devices and systems are needed that generally reduce undesirable movement of the implant relative to the bone for a required period of time, thereby substantially reducing formation of a fibrous tissue at the bone-implant interface. There is a particular need to temporarily and reliably stabilize uncemented joint implants to allow for bone ingrowth to occur on the surface of the implant and permanently stabilize the implant on the bone. At the same time, the devices and structures for adjunct stabilization are needed that would allow foregoing additional surgical procedures necessary to remove the devices from the body once the need in the stabilization, or fixation, passes, and, preferably, would allow substantially reducing the removal of the temporary fixation devices and structures during any required revision surgeries altogether, thus avoiding the risks of tissue damage associated with such removal and other surgical complications.
Moreover, systems and devices are needed that protect exposed bone tissue from undesired contact with particles and infectious agents after installation of the implant, thereby reducing the risks of infection and osteolysis. Additionally, devices and systems are needed that are sufficiently inert and stable in the human body, possess adequate mechanical and chemical properties to stabilize a joint implant in tissues and reliably remain in the tissues without causing undesirable side effects, such as degradation, undesired bone or soft tissue redistribution, or mechanical damage. The needed devices and systems should also provide adequate stabilization for a sufficient period of time to allow bone ingrowth to occur. Also desirable are systems that can serve as carriers for advantageous biologically active molecules, such as growth factors or antibiotics.
Also, stabilization devices and systems are desired that simplify removal of the implant during any required subsequent revision surgeries, thereby decreasing bone degradation and the risks of tissue damage associated with such removal, as well as other surgical complications. In general, temporary fixation devices and systems are needed that are readily available to a surgeon, easy to use, minimize tissue damage, and simplify any subsequently required surgical procedures. Temporary fixation devices and systems are needed that are versatile, allow for faster healing with fewer complications, require less immobilization, are easy to use and manufacture, and are inexpensive to produce and operate.